You want to build an ambulance out of wood? I don't think I can give you any advice other than "Good Luck."

The only real way to do it would be to buy an old van, paint it appropriately and add all the details. I can only imagine the decades of hard work it would take to replicate the same thing with lumber.

Like a 1930 model "A" or something like that.
Not many big bends in the shape. build a frame from wood, cover it with 1/4in. Luan which you can bend pretty well, usually.
Old metal from tore out furnace jackets or appliances works too, if it is to be metal?
You will need tin shears, right and left circle snips and pliers, draw out your pattern on poster board or cardboard first and tape this to the wooden frame for a test fit before marking and cutting the metal. Or make a steel frame from steel conduit?
Pop rivets and pop rivet guns are cheap and easy to use to attach the tin to tin and are easy to hide later.
My neighbor built a full-sized replica stagecoach using a small plastic model for the measurements and it looked so good!
I am starting to assemble and build a Ford Model "T", I have some real parts but mostly it will be a fabrication of mine. I have a 1/24 plastic model of the same car to get the measurments from.
Maybe you could come by some extremely bent up automotive sheet metal that could be placed next to your creation? you would only have to maybe build half or three-quarters of your ambulance, have the other side be represented by the actual damaged parts, painted to match the car you made.
Here is a list of the only Model "T" parts I have to work with: The frame.two wheels with tires, two running boards. Everything else I will be making.
It is tempting to make it a Model "T" speedster, pointy rear bodywork they could go 100MPH!

Cemetary Hill is the steepest hill here. A wagon load of October tourists went part way down that steep hill backwards when the wagon tongue broke three years after I said it would break.
Nobody got hurt.